What is the significance of a Cystatin C level of 0.95 and an estimated Glomerular Filtration Rate (eGFR) of 89, indicating mildly impaired renal function?

From the Guidelines

Your cystatin C level of 0.95 and estimated glomerular filtration rate (eGFR) of 89 indicate normal kidney function, as defined by the KDIGO guideline 1, which suggests that a GFR of 90 or above is considered normal. These values suggest your kidneys are filtering blood effectively, with the eGFR being very close to the normal threshold. Cystatin C is a protein produced by all nucleated cells at a constant rate and is freely filtered by the kidneys, making it a reliable marker of kidney function. Your level falls within the normal range (typically 0.5-1.0 mg/L). No specific treatment or medication is needed based on these values alone. To maintain good kidney health, continue staying well-hydrated, follow a balanced diet moderate in sodium, exercise regularly, and avoid nephrotoxic medications when possible. Regular monitoring of kidney function is recommended as part of routine health maintenance, especially if you have risk factors like diabetes, hypertension, or family history of kidney disease, as suggested by the KDIGO guideline 1. These current values can serve as a good baseline for future comparisons. According to the KDIGO guideline 1, the evaluation of albuminuria is also an important aspect of assessing kidney function, and clinicians should be aware of the determinants of albuminuria measurements to guide action. However, based on the provided information, your current kidney function is normal, and no immediate action is required. It is essential to note that the KDIGO guideline 1 recommends using validated GFR estimating equations and encourages clinicians to measure GFR by established clearance methods of glomerular filtration markers when more accurate ascertainment of GFR will impact treatment decisions. In your case, with a normal eGFR and cystatin C level, regular monitoring and maintenance of a healthy lifestyle are the best approaches to preserving kidney function.

From the FDA Drug Label

1. 3 Recommendations for Use in Renal Impairment Assess renal function prior to initiation of metformin hydrochloride tablets and periodically thereafter. Metformin hydrochloride tablets are contraindicated in patients with an estimated glomerular filtration rate (eGFR) below 30 mL/minute/1.73 m 2. Initiation of metformin hydrochloride tablets in patients with an eGFR between 30 to 45 mL/minute/1. 73 m 2is not recommended. In patients taking metformin hydrochloride tablets whose eGFR later falls below 45 mL/min/1.73 m 2, assess the benefit risk of continuing therapy.

2. 6 Renal Impairment Metformin is substantially excreted by the kidney, and the risk of metformin accumulation and lactic acidosis increases with the degree of renal impairment. Metformin hydrochloride tablets are contraindicated in severe renal impairment, patients with an estimated glomerular filtration rate (eGFR) below 30 mL/min/1.73 m 2

The patient's eGFR is 89, which is above the contraindicated threshold of 30 mL/min/1.73 m^2. According to the drug label, metformin can be used in patients with an eGFR above 45 mL/min/1.73 m^2, and the patient's eGFR is within the acceptable range 2 2.

Key points:

• The patient's eGFR is 89, which is above the threshold for contraindication.
• Metformin can be used in patients with an eGFR above 45 mL/min/1.73 m^2.
• The patient's eGFR is within the acceptable range for metformin use.

From the Research

Cystatin C and eGFR

• Cystatin C is a reliable and accurate marker of kidney function, particularly helpful in patients for whom creatinine is an inadequate marker 3, 4.
• The 2012 Kidney Disease Improving Global Outcomes (KDIGO) guidelines recommended using cystatin C to confirm the diagnosis of chronic kidney disease (CKD) determined by creatinine-based estimated glomerular filtration rate (eGFR) 4.
• A study found that a large difference between creatinine- and cystatin C-based eGFR is associated with adverse cardiovascular outcomes in patients with CKD 5.

Clinical Considerations

• Cystatin C improves sensitivity and specificity of chronic kidney disease diagnosis, improves detection of harmful acute and chronic changes in kidney function, and improves precision of treatment eligibility and safety 6.
• The National Kidney Foundation (NKF) and American Society of Nephrology (ASN) Joint Task Force recommended increasing availability and clinical adoption of cystatin C to assess kidney function 4.
• A study suggested that careful monitoring of cardiovascular disease is needed for patients with a higher difference between creatinine- and cystatin C-based eGFR 5.

Cystatin C in Clinical Practice

• Cystatin C should be routinely available for estimating kidney function, particularly in vulnerable individuals such as older adults, females, non-White individuals, and those living with multiple medical conditions 6.
• Better education, curiosity, and motivation among nephrologists could substantially improve the availability and utilization of cystatin C 6.
• A study found that serum cystatin C level performs similarly to serum creatinine level, serum urea nitrogen level, and urine output for predicting dialysis requirement or in-hospital death in patients with acute kidney injury 7.